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I am trying to make an electronic component that will do the following:

Vc is the control voltage, it can change the current thru the device.

I need Vx independent of the the current, that means current is fixed whatever the Vx is.

How should I do it?

I was thinking about a MOSFET but, the change in drain and source voltage could change the current.

enter image description here

JRE
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kintaro
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    You need to put in a feedback loop to compensate for everything you mentioned plus temperature effects etc. – vir Aug 15 '22 at 20:35
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    @Antonio51 also called "voltage-controlled current source" like it says in the title. – user253751 Aug 15 '22 at 21:19
  • Your diagram is for a high-side current source. A low-side current source is easier. If you describe your problem in more detail, we may be able to tell which is necessary. Here is a high-side current source. https://www.ti.com/lit/an/slaa867/slaa867.pdf Realize that it is only going to work over a range of input voltages. – Mattman944 Aug 15 '22 at 21:33
  • Here are some questions you must answer before anyone can give you a definitive answer that would work for you: 1) What maximum current do you want for your load? 2) What range should we expect for Vx? 3) What is your desired Vc-to-I relationship? 4) Where and how is Vc produced? 4) Do you have a stable voltage source to power this circuit other than Vx? 5) What is the load? – Simon Fitch Aug 16 '22 at 11:41

3 Answers3

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Here is an example of such a circuit.

Some specifications must obviously be done ...
as max current, max voltage, voltage control window levels, and some others ...

enter image description here

Antonio51
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It's easy to make a voltage controlled current source using op-amps, but here is a simple circuit using an NPN and PNP transistor. It's pretty linear once you get past the offset due to the the base-emitter voltage.

Current Source NPN and PNP

Diode D1 compensates for base-emitter voltage of Q2.

You can also use a PMOS device:

Current Source NPN and PMOS

And an NJFET provides a linear response almost down to zero input and output, as well as output within 1 volt of supply rail.

Current Source NJFET and PNP

PStechPaul
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Use a resistor in the source and an opamp to regulate the I*R drop across that R by driving the gate of the FET.

jp314
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